A cGMP phosphodiesterase (PDE) was extracted from bovine rod outer segments and purified by chromatography on AcA34. cGMP is the preferred substrate for this PDE. cGMP binding sites were studied using photolabelling techniques and direct binding studies. IBMX which inhibited cGMP hydrolysis in a competitive fashion did not interfere with (3H)cGMP binding or photolabelling with (32P)cGMP. cAMP was a very ineffective competitor for (3H)cGMP binding; only (32P)cGMP, not (32P)cAMP or 8-azido (32P)cAMP formed photoadducts with the ROS cGMP PDE. In general, several cAMP derivatives were not as effective as cGMP or 8-Br cGMP in inhibiting cGMP hydrolysis or competing for (3H)cGMP binding sites. 8-Chloropurine riboside cycle monophosphate was, however, more effective in inhibiting (3H)cGMP binding thin hydrolysis. These studies suggest that distinct sites with differing topography may be involved in binding and hydrolysis of cGMP by the ROS cGMP PDE.